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Back Electrical Electrical Engineering Dictionary Electrical Equipment Grid Station - Control Protection & Equipment of Grid Stations

Grid Station - Control Protection & Equipment of Grid Stations

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Grid stations plays a vital role in the distribution of Hydroelectric power form their generation units (dams) to the remote city and suburb areas, through a proper distribution system called Grid Stations or shortly GSS.

There are different kinds of Grid Stations such as

  • ` 66 KV Grid Station
  • ` 132 KV Grid Station
  • ` 220 KV Grid Station
  • ` 500 KV Grid Station

In this article only 66 KV and 132 Grid Stations are discussed with practical examples of Local Grid Stations in Khyber Pakhtunkhwa Province of Pakistan.

Electric Supply Grid Station WAPDA, Peshawar

Switch Gear System

To connect the step-up transformers to 132KV bus bar a disconnecting device is needed, which consists of medium voltage circuit breakers, medium voltage disconnecting switches and the corresponding metering and protection equipment.

It is the combination of electrical disconnects, fuses and/or circuit breakers used to isolate electrical equipment.

132 KV SwitchGear

Metal clad switchgear Panel shall be indoor type. The switchgear shall have all necessary instruments & accessories including Vacuum type Circuit Breaker (VCB).

VCBs are preferred due to very fast switching and quick arc quenching process. These breakers are almost maintenance free. The current/voltage transformer (CT/PT) shall be used for metering and protection duties.

0.4 KV Switchgears & Switch boards

It is proposed to use a 400 V switch gear scheme for this HPP to supply power to station auxiliaries. The AC supply of 400 V can be obtained from the 132kV bus bar through two 300kVA transformers, keeping one transformer as standby.

The station service transformers shall supply to the unit switchboard and common services switchboard through main switchboard.

Cables

The cables shall be used to connect the generators with the transformers, switchgears, transmission feeders and auxiliary transformers. Special cables shall be used for specific jobs such as control, measurement and protection. Specific use of these cables is described as under:

Power Cables

The power cables to be used between the generators, transformers and their respective 0.4 kV, 132KV switchgears shall be as per IEC/WAPDA standards.

Control Cables

Control cables are used for protection, instrumentation, control and communications. These shall be of stranded copper conductors or armored cables. Control cables will be laid in perforated steel trays or at the bottom of concrete trenches.

The cable trays shall be mounted on the floor underneath the local control area, to have short connections between the different control panels, distribution boards, equipment and other auxiliary installations. The cable lengths are kept to a minimum, as the control area and distribution equipment is located within the powerhouse building.

Control Protection & Instrumentation Equipment

Instrumentation & Control

Indicating, measuring and control instruments have been considered necessary for the major components of Power House .The transducers shall be installed where necessary. All indications shall be visible in the main control room.

The control of the single unit shall be local and remote able for auto or manual selection. It is necessary to provide a separate control desk and mimic panel that shall be located in the main control room and include the most important remote control functions. Because of standard practice, a control voltage of 220 V is preferred.

Protection

Protection of the power plant and equipment is necessary for safe and reliable operation. Turbines, generators transformers and transmission lines shall be provided with an adequate protection relay system. Typical unit trip functions are given as follows:

Generators & Turbines

  • ` Time over current
  • ` Temperature (thermo detector)
  • ` Negative sequence current
  • ` Loss of input(Anti-motoring)
  • ` Loss of synchronism
  • ` Backup over current
  • `Bearing insulation
  • ` Governor oil level low
  • ` Governor oil pressure low

Step Up Transformers

  • ` Winding temperature high (for hot spot)
  • ` Oil temperature and level control
  • ` Buchholz protection
  • ` Three phase over current protection as back up device.
  • ` Three phase unit block differential over current protection.
  • ` Neutral point earth fault current protection.( common for all units)
  • ` One lightening arrestor on each phase on high voltage side.

132 KV Transmission Line

  • Over current protection
  • Earth fault protection

Station Batteries System

The purpose of the station battery system is to provide a safe and reliable supply of power to all primary functions. The system is almost independent of all other power supply systems and ensures reliable execution of the control functions, both for selected normal operation and during possible fault conditions. DC batteries shall be installed in the powerhouse for protection, emergency power, alarms and indications. The battery installations proposed for this HPP are as follows:

Two sets of 220 V batteries shall serve the whole power station. The batteries shall have their own battery charger. Under normal conditions, DC supply shall be obtained through an AC to DC rectifier/converter and in case of failure of AC supplies; DC batteries shall be automatically buffering the DC system. Capacity of battery shall be finalized at detail engineering stage.

Emergency Diesel Generator Set

It is proposed to incorporate a stand-by diesel generator set. The diesel generator should be capable to supply auxiliary power at least for one unit. The recommended ratings are 250KVA, 400V and will be located at a convenient location.

Telecommunication

In view of a successful operation of this HPP, a complete communication system shall be developed. Within the area of the powerhouse and outside, such as switch yard, machine hall, control room etc an intercom telephone system should be provided where local communication is required. For communication with the Grid Station and Load Dispatch Center, two telephone lines from local telephone company such as PTCL (Pakistan Tele Communications Limited) should also be provided in the control room and in the offices. To interchange data between the power station and the national grid, PLC system is also recommended.

Grounding System

Metallic parts of all the electrical equipment in or around the powerhouse shall be connected to the grounding system for safety of the personnel and equipment. It shall use stranded bare copper conductor in the concrete floor slabs. Ground rods shall be driven in soil surrounding the powerhouse. The number of electrodes shall be determined during the final design stage, keeping in view the soil resistivity and touch/step voltage .The grounding grid shall be capable to sustain without damage heavy currents during short-circuits for specified time.

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